Method prepare_key

jwt/algorithms.py:325–357

(self, key: str | bytes)

        self.hash_alg = hash_alg

    def prepare_key(self, key: str | bytes) -> bytes:
        key_bytes = force_bytes(key)

        if len(key_bytes) == 0:
            raise InvalidKeyError("HMAC key must not be empty.")

        if is_pem_format(key_bytes) or is_ssh_key(key_bytes):
            raise InvalidKeyError(
                "The specified key is an asymmetric key or x509 certificate and"
                " should not be used as an HMAC secret."
            )

        # Defense against algorithm-confusion attacks: an attacker with
        # control over the token header can force this code path by setting
        # alg=HS*, and HMACAlgorithm is the only algorithm that accepts
        # arbitrary bytes as a valid secret. Other algorithms reject
        # non-key-shaped input naturally. Even a symmetric (kty=oct) JWK
        # should be loaded via PyJWK / from_jwk rather than fed as raw JSON
        # bytes (whose contents are not the secret material).
        stripped = key_bytes.lstrip()
        if stripped.startswith(b"{"):
            try:
                jwk_obj = json.loads(key_bytes)
            except ValueError:
                jwk_obj = None
            if isinstance(jwk_obj, dict) and "kty" in jwk_obj:
                raise InvalidKeyError(
                    "The specified key looks like a JWK and should not be "
                    "used directly as an HMAC secret. Load it via "
                    "PyJWK / HMACAlgorithm.from_jwk first."
                )

        return key_bytes

    @overload
    @staticmethod